an imperfection in the clouds. I've seen similar photographs by other ahem.... NERRRRRRRDS where the red spot is unmistakable.

I basically filmed a 30 second clip of Jupiter, and then used some software which splits the film into individual frames and layers them on top of each other. You select the best frames, get rid of the rubbish ones, and stack them. I was astonished by the detail in the results, through the eyepiece it just looks like a vague orange blob.

But yeah, I reckon that dark bump is just dark cloud which has been enhanced by the layering. If it was the Red Spot you'd be certain of it.